1. Field of the Invention
The present invention relates to a light emitting device in which a unit for supplying current to a light emitting element and a light emitting element are provided in each of plural pixels, and more particularly a device substrate corresponding to a form of a light emitting element which is not yet completely fabricated in the process of manufacturing the light emitting device in which a unit for supplying current to a light emitting element is provided in each of a pluralities of pixels.
2. Description of the Related Art
Next, a pixel structure of a general light emitting device and the drive thereof will be described briefly. A pixel shown in FIG. 10A has TFTs 80 and 81, a storage capacitor 82 and a light emitting element 83. Note that, a storage capacitor 82 need not always be formed.
In the TFT 80, a gate electrode is connected to a scanning line 85, one of a source region and a drain region of the TFT 80 is connected to a signal line 84 and the other is connected to the gate electrode of the TFT 81. In the TFT 81, a source region is connected to a power source line 86, and a drain region is connected to an anode of the light emitting element 83. The storage capacitor 82 is provided so as to retain voltage between the gate electrode and the source region of the TFT 81. The power source line 86 and the cathode of the light emitting element 83 are respectively applied with predetermined potential from the power source and have mutual potential difference.
Note that, a connection means an electrical connection in this specification, if there is no specific description.
When the TFT 80 is turned on by potential of the scanning line 85, potential of a video signal input to the signal line 84 is given to the gate electrode of the TFT 81. In accordance with the potential of the input video signal, a gate voltage (a voltage difference between the gate electrode and the source region) of the TFT 81 is determined. Then, drain current that flows in accordance with the gate voltage is supplied to the light emitting element 83 and the light emitting element emits light in accordance with the supplied current.
A pixel structure in general light emitting device, which is different from FIG. 10A is shown in FIG. 10B. The pixel shown in FIG. 10B has TFTs 60, 61 and 67, a storage capacitor 62, and a light emitting element 63. It is noted that the storage capacitor 62 is not necessarily provided.
In the TFT 60, a gate electrode connected to a first scanning line 65, one of a source region and a drain region is connected to a signal line 64, and the other is connected to a gate electrode of the TFT 61. In the TFT 67, a gate electrode is connected to a second scanning line 68, one of a source region and a drain region is connected to a power source line 66, and the other is connected to a gate electrode of the TFT 61. In the TFT 61, a source region is connected to the power source line 66 and a drain region is connected to an anode of the light emitting element 63. The storage capacitor 62 is provided in order to keep voltage between the gate electrode and the source region of the TFT 61. The power source line 66 and the cathode of the light emitting element 63 are respectively applied with predetermined potential from the power source and have mutual potential difference.
When the TFT 60 is turned on in accordance with potential of the first scanning line 65, potential of a video signal input to the source line 64 is given to the gate electrode of the TFT 61. In accordance with the potential of the input video signal, a gate voltage (a voltage difference between the gate electrode and the source region) of the TFT 61 is determined. Then, drain current of the TFT 61 that flows in accordance with the gate voltage is supplied to the light emitting element 63 and the light emitting element 63 emits light in accordance with the supplied current.
In addition, in the pixel shown in FIG. 10B, when the TFT 67 is turned on in accordance with potential of the second scanning line 68, potential of the power source line 66 is given to the both the gate electrode and the source region of the TFT 61, and therefore the TFT 61 is turned off and the light emitting element 63 is forced to finish emitting a light.
Now, in many electroluminescence materials in which electroluminescence can be obtained by impressing electric field, luminance of red luminescence is generally low, compared with luminance of blue or green luminescence. In the case of applying a light emitting element using an electroluminescence material with such a characteristic to a light emitting device, luminance of red in a displayed image is likely to be naturally low.
Especially, in the case of a color display method of forming three kinds of light emitting elements corresponding to R (red), G (green), and B (blue) respectively, it is difficult to control a balance of white color.
It has been conventionally carried out the way to use orange light with a little wavelength than red light as red light. However, with this way, an image to be displayed as red image is displayed as orange as a result and then, the purity of red light is low and.
Then, as a means for controlling the balance of luminance of red, blue, and green luminescence, it is generally employed to make current supplied to a pixel different from each other in displaying RGB (red, green, and blue). Specifically, it is possible to make the current supplied to a pixel different and keep the balance of white light when potential difference between a power source line and cathode of a light emitting element is made different for each of RGB. (ref. Japanese Patent Laid Open No. 2001-159878. 5th page)